1. Field of the Invention
The present invention relates to a camera having an automatic focusing device and, more particularly, to a camera having an automatic focusing device with a plurality of focus detectors employing different distance measuring methods.
2. Related Background Art
Compact cameras and the like for taking pictures using a roll of photographic film have been in common practical use. Such cameras have an automatic focusing device with distance measuring means for measuring the distance of an object (hereinafter referred to as "object distance"), for automatically adjusting the focus by moving a photographic lens optical system to a predetermined position in accordance with the result of distance measurement performed by the distance measuring means.
Various methods for distance measuring means of automatic focusing devices commonly used in conventional compact cameras and the like (hereinafter simply referred to as "cameras"), e.g., passive method, active method and the like have been proposed and put in practical use.
An active distance measuring method is constituted by light projection means for radiating (projecting) a beam of light, e.g., infrared light upon an object and light, receiving means or the like for receiving reflection of the light. A predetermined beam of light is radiated (projected) upon a desired object using the light projection means, and light reflected thereby is received by the light receiving means which is a position sensitive device (PSD) or the like. Triangulation is used to calculate the distance of the object from the angle defined by the emitted beam and the incident beam.
Passive distance measuring methods are generally categorized into TTL methods which are primarily used in the so-called single-lens reflex cameras or the like and ambient light methods which are primarily used in the so-called compact cameras and the like. Both of the TTL methods and ambient light methods are constituted by pupil split means for performing pupil splitting on a beam of light from an object such that two images can be formed therefrom and light receiving means such as a charge coupled device (CCD) for receiving two beams of light split by the pupil split means. In this case, an image of each of the two beams of light from the object split by the pupil split means is formed on a light receiving surface of the light receiving means (CCD), and a difference between the phases of the two images is calculated to measure the distance between the two images.
In a TTL method, a defocusing amount of a photographic lens is calculated in accordance with the result of the measurement of the distance between two images obtained as described above. In an ambient light method, the distance of an object is calculated based on triangulation from the result of measurement of the distance between two images thus obtained.
Distance measuring means utilizing those distance measuring methods may not be able to provide reliable results of distance measurement depending on objects. For example, reliable results of distance measurement may not be obtained by using an active type distance measuring means in instances wherein the object of interest is in a backlighted state, wherein the object itself emits intense reflected light, wherein an intense reflecting member or the like is in a screen and wherein the object is at a too long distance (a too long object distance) to be reached by the light projected for distance measurement.
Accurate results of distance measurement may not be obtained by using a passive type distance measuring means in instances wherein the object of interest has low luminance or low contrast and wherein the environment around the object is an environment of low luminance.
Under such circumstances, in order to cancel the shortcomings of those means, proposals on so-called hybrid AF type distance measuring means including distance measuring means based on both of the above-described distance measuring methods, for finding the distance of an object of interest by switching the distance measuring methods appropriately depending on the object have been disclosed in, for example, Japanese examined patent publications No. H3-78602 and No. H3-78603 and Japanese unexamined patent publications No. S63-259521 and No. H7-168090 and have been put in practical use. Further, compact cameras and the like with such hybrid AF type distance measuring means have been also put in practical use recently.
When one of distance measuring means of such hybrid AF type distance measuring means (hereinafter referred to as "hybrid AF means") can not detect the focus of an object of interest, the distance measurement is performed by switching the distance measuring means to the other. This makes it possible to obtain photographic results with sufficiently good image quality, for example, when the object is stationary because reliable focusing can be performed regardless of the conditions of the object and the photographic environment.
Therefore, automatic focusing devices having such hybrid AF type distance measuring means are spreading as distance measuring means for automatic focusing devices used in recent compact cameras and the like.
However, automatic focusing devices utilizing hybrid AF type means as described above have a problem in that it is difficult to determine which of a plurality of distance measuring means based on different distance measuring methods is adequate to use for photographing, for example, an object moving at a high speed. The above-cited publications pay no consideration on this point and even make no mention on the presence of such a problem.
So-called moving object predicting AF means and the like which reflect predicted movement of an object on the result of distance measurement have been put in practical use as means for reliably focusing a moving object in conventional single-lens reflex cameras and the like.
However, the above-cited publications make no mention on the use of the above-described moving object predicting AF means in each of the distance measuring means. It is therefore thought that focus detection on a moving object utilizing the means according to the above-described publications will not provide reliable results of distance measurement (results of focusing).
In general, it is considered that the use of the above-described passive method is suitable for photographing of a moving object. The reasons are as listed below.
(1) When a moving object is photographed, the distance of the object tends to increase. Specifically, since it is difficult to photograph an object moving at a high speed, e.g., a train or automobile at close range because it is dangerous to approach it to photograph, the object distance inevitably becomes long. Since the passive method is thus suitable for distance measurement of an object at a long distance, the passive method is preferably used to photograph a moving object.
(2) In the case of the active method in which light such as infrared light is projected upon an object, it is more difficult to reliably project a beam of light upon a desired position of a moving object than projecting light upon a stationary object depending on the size of the object (the problem of occurrence of so-called missing spots). Specifically, it is difficult to continuously keep a moving object within a predetermined range for distance measurement and to continuously project a beam of illumination light upon a predetermined position of an object of interest.
(3) In general, techniques for passive type moving object predicting AF means have been accumulated in a far greater amount, and it is therefore easier to implement moving object predicting AF means using a passive method.
For the above and other reasons, the use of distance measuring means utilizing a passive method is considered more suitable for a distance measuring operation on a moving object.
The present invention has been made taking the above-described points into consideration.